Electrostatic coalescer device and use of the device

ABSTRACT

A device, located in a vessel having an inlet and at least one outlet, through which a mixture of fluids flows, for promoting electrostatic coalescence of a first conductive fluid emulsified in a second fluid. The device includes a number of tubular electrostatic coalescer elements extending in the flow direction which are arranged in a matrix arranged over the cross sectional area of the vessel, and an applicator operative to apply an electrical field to the fluids flowing through the coalescer elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Norwegian patent application No.20015454 filed Nov. 7, 2001 and is the national phase under 35 U.S.C.§371 of PCT/IB02/04657 filed Nov. 6, 2002.

TECHNICAL FIELD

The present invention relates to an electrically energised device foruse in the separation of a first conductive fluid, emulsified in asecond fluid. The invention finds its main application in the oilindustry. It is particularly advantageous in off-shore applications inwhich equipment is arranged for the purpose of promoting or effectuatinga pre-separation of water from oil, or a water droplet enlargement,before an extracted emulsion comprising oil and water is furtherconducted to a subsequent settling tank for gravitational settling.

BACKGROUND OF THE INVENTION

Fluids produced from an underground formation, is usually a three-phasemixture of water, oil and gas, in which at least some of the water isemulsified in the oil. This mixture of fluids is usually separated intoits phases downstream of the wellhead, in order to be deliverable topipelines for further distribution. A possible method for performingsuch a separation is to use a three-stage process, with two gravityseparator tanks followed by a last separator tank including anelectrostatic coalescer. The gravity separators usually include a flowstraightener at the inlet. The purpose of the flow straightener istwo-fold; to equalize the flow regime across the section of the tank,and to affect a mechanical coalescing of water droplets prior to theseparation by gravity, in order to increase the efficiency of theseparation process.

The problem with this multistage process is that separation is mosteffective at the first stage, where there is a large percentage of waterto be separated out. At the last stage, the percentage of water isrelatively low, which means that an electrostatic coalescer must beintroduced in this gravity separator tank.

Still, the efficiency is low, which means that the fluids have to remainin the separator tank for a long time in order to allow the phases toseparate out. Thus, the tank has to be large, e.g. about four meters indiameter and 20 meters long. It is inconvenient to use a tank of thissize at the production site, especially offshore where the availablespace is limited.

Prior Art

U.S. Pat. No. 4,469,582 describes an electrically enhanced inclinedplate separator in a downstream compartment connected to an electricalsystem to generate electric fields within the passages of the separatorto coalesce and separate a polar is liquid (water) from a non-polarliquid (oil). The separator generally consists of an array of parallelflat, or corrugated, plates (electrodes) through which the processedmixture of liquids passes. Each plate is made of two sections. The firstsection is made of en electrically conductive material. The downstream,second section is made of a non-conductive material.

U.S. Pat. No. 4,919,777 describes a treater for electrostatic/mechanicalseparation of brine from oil during longitudinal flow through ahorizontally elongate tank, wherein coalescer elements are provided forenhancement of the de-emulsification process. The emulsion is directedthrough electrical fields where the brine droplets take on an electricalcharge, then moved through an electrically grounded coalescing elementcomprising a multiplicity of longitudinally-extendingdownwardly-inclined open-ended tubes arranged in a bundle-fashion.

A drawback of the separators disclosed in U.S. Pat. Nos. 4,469,582 and4,919,777 is that they both use bare electrodes. A coalescer with bareelectrodes in contact with the fluids would not withstand the conditionof being flooded with water. The saline water is conductive and willshort-circuit the electrodes, rendering the electrical systeminoperative. With bare electrodes, water content of 10% is normallyregarded as the limit. Also, such bare electrodes would be subjected tocorrosion due to the emulsion. Thus, these can only be used late in achain of settling tanks, where the water content has been brought downbelow the 10% limit.

BRIEF DESCRIPTIONS OF THE INVENTION

It is an object of the present invention to provide a device in aseparator vessel, such as a gravity separator tank, that enhances theseparation of the individual phases is in a mixture of fluids, and atthe same time to ensure a proper plug-flow in the separator(s).

Another object of the invention is to provide such a device thatefficiently can coalesce a conductive fluid in an emulsion, such aswater in a water-oil emulsion.

Another object of the invention is to provide a device that contributesto a reduction of the overall size of the separator vessel, and/orultimately can reduce the number of vessels in a multi-stage process.

Still another object of the invention is to provide a device that easilymay be installed in existing gravity separators, i.e. through themanhole.

These objects are achieved by a device and arrangement employing thedevice.

In essence, the invention consists of a combination of a flowstraightener and a multitude of coalescer elements.

Expressed in another way, the invention comprises a stack of coalescerelements which at the same time work as a flow straightener.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to theappended drawings, in which:

FIG. 1 gives a schematic overview of a three-stage separation process,according to prior art.

FIG. 2 shows an embodiment of the invention.

FIG. 3 shows another possible embodiment of the present invention.

FIG. 4 shows an embodiment of the invention using inductive energytransfer to the individual coalescer elements.

FIG. 5 shows another embodiment of the invention using capacitive energytransfer.

FIG. 6 is a diagram showing the separation effect of a conventionalgravity separator, with the level of the separated phases shown as afunction of time.

FIG. 7 shows a similar diagram after the installation of a deviceaccording to the present invention.

FIG. 8 shows an embodiment in which the inventive device includes anadditional perforated plate mounted upstream of the matrix of coalescerelements.

FIG. 9 shows an arrangement in which several devices according to thepresent invention are mounted in series.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic drawing of a separator system, normally placeddownstream the wellhead. Some of the requirements for such a separatorsystem are to bring down the pressure and temperature of the oilproduction stream from the well (typically 60 bar and 60° C.,respectively) to typically 1 bar and 15° C. for export. In addition, theexported oil should contain less than 0.5% water. To meet theserequirements, a three-stage process can be used, comprising threegravity separator tanks 1, 3, 5. A well stream containing oil, water andgas is entering the first separator 1. The separator 1 has a flowstraightener 2 at the inlet. The flow straightener acts to ensure properplug-flow in the main body of the tank, i.e. to distribute andhomogenise the multi-phase flow over the cross section of the tank. Theflow straightener is normally a perforated plate with a certain pressureloss to evenly distribute the volume flow. Typically, the perforation is15-25% of the cross sectional area with a diameter of the holes in theregion 10-30 mm. It is common to use two such plates, with the holessomewhat displaced radially. The rather narrow holes will introduce someturbulence in the flow. This will act to coalesce the water droplets andthus will speed up the subsequent separation of the fluids. The fluidsleaving the first separator will typically contain about 2-30% wateremulsified in the oil.

The next separator 3 is of a similar construction to the first separator1, and will bring down the water content to approximately 1-10%.

It now becomes harder to remove the remaining water and the separationprocess in the last separator 5 is therefore slow, typically with thefluids retained in the tank for about 10 minutes, compared to about 3minutes in the previous separators.

The idea behind the present invention is to combine the features of aflow straightener with the inline electrostatic coalescer. This is done,as shown in FIGS. 2 and 3, by applying a stack of individualelectrostatic coalescers, each comprising a tubular channel throughwhich the emulsion is to flow, and with external electrodes on theoutside of the channels for applying an electrical field to theemulsion, instead of the traditional perforated plates for flowstraightening. Thus, the separation will be enhanced in addition toensure proper plug flow in the separators. This new flowstraightener/coalescer is installed in the first gravity separators, 1,3 preferably at their inlets. Here, the effect of the separation processis at the greatest, and the inventive flow straightener/coalescer willimprove the effectiveness of these separators 1, 3. The effectiveness ofsaid separators can thus substantially be improved so that the oilleaving the second separator contains less than 0.5% water, i.e. meetsthe requirements of the customers. Then, the last separator 5 mayultimately be dispensed with.

FIG. 2 shows a first embodiment of the inventive combined coalescer andflow straightener. The device comprises a number of small-scalecoalescer elements 21 organised in a matrix over the cross section ofthe separator or vessel. Each coalescer element 21 comprises apreferably tubular channel 24 made of an insulating material and a firstelectrode 25 and a second electrode 26 attached to the channel 24 on theouter surface thereof. The wall of the channel 24 separates theelectrodes 25, 26 from immediate contact with the emulsion or fluidsflowing through the channel. The individual channels define rows, and anumber of electrodes are arranged as sheets or plates separatingadjacent rows. Thus the electrodes 25 and 26 are common for all channelsin each row, and each electrode is also common for the rows on oppositesides thereof. The first electrodes 25 are of opposite polarity comparedto the second electrodes 26.

FIG. 3 shows an alternative embodiment of the inventive device, in whicheach separate channel 34 is surrounded by a pair of helical windedelectrodes 35, 36, having opposite polarity as disclosed in theco-pending Norwegian patent application no. 2000 2383. However, whilethis embodiment has proved to be more efficient than the firstembodiment in separating the fluids, this is a more complicated designto implement, and the first embodiment will thus be the preferredembodiment.

One additional requirement of the inventive device is that it shouldwithstand both being flooded with water, i.e. being partly or completelyfilled with a water continuous phase, and going dry, i.e. being partlyor completely filled with gas. To meet this requirement, insulatedelectrodes must be used. In the embodiments shown in FIGS. 2 and 3, theelectrodes are external to the tubes, and thus no problems arise for anemulsion with high water content. As mentioned earlier, the invention isintended to be installed upstream in the separation process, even in thefirst separator tank where the water content is high. Then it will be aprerequisite to use insulated electrodes in order to cope with emulsionscontaining in excess of 10% water.

The inventive device is intended to be used in offshore installationsboth topside and subsea, i.e. at the seabed. In these harshenvironments, it is preferred to feed the device from a low-voltagepower source. The low-voltage line, typically 400 volts AC, is conductedthrough the wall of the vessel into a central module 41, 57 with one ormore integrated transformers, see FIGS. 4 and 5. The matrix of coalescerelements are preferably made as coalescer modules. In cases where theinventive device is applied to the retrofit market, the size of eachmodule must not exceed the size of the manhole of an existing separatortank.

In the present invention, the electrical energy is transferred withoutgalvanic contact between the central module 41, 51 and the correspondingcoalescer modules 42, 52 in order to energise the electrodes of thecoalescer modules. In the embodiment shown in FIG. 4, inductive couplingis used to energise the coalescer electrodes. The central module 41comprises a number of primary transformer windings 45 on half cores 44.Adjacent to the central module 41 are mounted a number of coalescermodules 42, each including a corresponding secondary winding 47 on ahalf core 46. Each half core 44, 46 is mounted near the wall of therespective module 41, 42, protected by an insulating layer. Thecoalescer modules 42 are mounted on the central module 41 in theseparator vessel, and half cores 44, 46 and windings is 45, 47 willtogether form a complete transformer. The transformers are fed with lowvoltage AC from the line 43 and supply the electrodes in the coalescermodules 42 with high voltage. This inductive coupling allows all thoseparts of the construction that are carrying electricity to be completelyembedded in oil/water compatible insulated materials and thus protectthem against the harsh environment present in the separator. The modulescan e.g. be moulded in epoxy. The choice of proper materials for thisapplication is further described in NO 2000 2383.

Another way of achieving the energy transfer without galvanic contactbetween the central module and the neighbouring electrode modules is touse capacitive coupling, see FIG. 5.

The embodiment shown in FIG. 5 comprises a central module 51 surroundedby coalescer modules 52. The central module 51 includes a transformerfed with low voltage AC. High voltage from the transformer secondariesis fed to plates 53 located parallel to and in close proximity to themodule wall.

In the coalescer modules 52, corresponding plates 54 are locatedparallel to the wall adjacent to the central module 51. Pairs of plates53, 54 located in the central module 51 and in the coalescer modules 52form capacitors, allowing energy transfer from the central module 51 tothe electrodes in the neighbouring coalescer modules 52. However, as inthe case of the inductive coupling there are many ways known to thoseskilled in the art to technically achieve the capacitive energy transferbetween the central module and the neighbouring coalescer modules. Theone shown in FIG. 5 is just one example presented to reveal the conceptof using capacitive coupling to allow contactless transfer of the energyrequired to energise the electrodes in the coalescer modules.

Yet another way of supplying the electrodes of the coalescer elementsbeing arranged in one or more modules with high voltage is to mould acomplete transformer into each individual coalescer module, in thispreferred embodiment the transformer of the module has a separate lowvoltage winding terminal accessible from the outside and at the sametime the high voltage secondary winding insulated from the fluids.

In the embodiments shown in FIGS. 2-5, each individual channel cantypically be 5-30 mm in diameter and 100-500 mm in length.

FIG. 6 shows the results of some experiments with a very stablewater-oil emulsion containing 10% water. The emulsion is allowed tosettle by gravity in a tank. The line with square points in FIG. 6 showsthe position of the interface between the water or water continuousphase and the emulsion, while the line with triangular points shows thecorresponding interface between the emulsion and the oil or oilcontinuous phase. As the diagram shows, with a mixture containing 10%water, no separation is observed for the first five hours. Theseparation continues very slowly from then on, and no visible interfacebetween the oil continuous and water continuous phase is observed.

FIG. 7 shows the results after the introduction of the inventive devicein the settling tank. In the example, the retention time in the combinedcoalescer and flow straightener device is about four seconds. As can beseen from the figure, a pronounced interface between the oil continuousand water continuous phases is obtained, as indicated by the line withcircular points, and the separation process is almost completed afterabout one hour. The reader should be aware of that this is a laboratoryset-up in a small scale, which is why the observed settling times do notcorrespond to the times mentioned earlier for a full scale productionfacility.

For a conventional flow straightener in a settling tank it isadvantageous that the fluids obtain a significant level of turbulence topromote droplet collision rate and thereby coalescence, and providesufficient shear forces to break down any dense emulsion layer. This isnormally obtained by an evenly distributed pressure loss throughout theentire cross section area of the tank. This also promotes the desiredplug flow pattern in the settling tank.

In a device according to the invention, the turbulence may be increasedby restricting the flow area at the inlet of each channel, or by anyother method, as well known by the artisan.

The tubular channels can also be arranged inclined downwards in thedirection of flow. This promotes a self-cleaning effect, to avoidparticles such as sand carried by the fluids being sedimented in thechannels and possible clogging them.

An arrangement employing the present invention is shown in FIG. 8. Acoalescer device 82 is mounted near the inlet of a gravity separatortank. An additional perforated metal plate 81 is mounted upstream of thecoalescing device 82. The plate is connected to ground (i.e. the body ofthe tank), or more correctly to the mean value of the potentials U+ andU− applied to the coalescer electrodes. This arrangement createsmultiple inhomogenous electric fields in the area 83 between the edgesof the electrodes and the perforated plate, which significantly extendsthe active zone of the coalescing device. The plate will also act todistribute and homogenise the flow over the cross section of the tank.

In FIG. 9 is shown another arrangement involving the invention. In agravity separator tank, coalescing devices 92 a, 92 b, 92 c are arrangedin series in the flow direction. In the subsequently arranged devices 92a, 92 b, 92 c, coalescer electrodes at the same vertical position areconnected to opposite polarity. This acts to create highly inhomogenouselectric fields in the area between the subsequently arranged devices,which extends the active zone of the coalescer device.

Since the invention is particularly applicable for emulsions comprisingoil and water, it has been described with reference to such anapplication. However, it should be understood that the invention isapplicable to all kinds of applications in which there is possible toensure a proper plug flow of fluids in an emulsion and at the same timecoalesce a first polar fluid emulsified in a second nonpolar fluid bymeans of an electric field applied to the emulsion.

1. A device, located in a vessel having an inlet and at least oneoutlet, through which a mixture of fluids flows, for promotingelectrostatic coalescence of a first conductive fluid emulsified in asecond fluid, the device comprising: a plurality of tubularelectrostatic coalescer elements extending in a flow direction, eachcoalescer element comprising an insulating tubular channel through whichthe mixture of fluids is to flow, and each coalescer element furthercomprising a pair of interacting external electrodes arranged outsideand adjacent to the insulating tubular channel, the interacting pair ofelectrodes have opposite polarity, are separated by a gap, extend inparallel in the length direction of said channel and are electricallyinsulated from fluids outside of the channel, wherein the plurality ofcoalescer elements are arranged in a matrix of coalescer elements over across sectional area of said vessel such that the tubular channels areparallel to each other, wherein the matrix of coalescer elements formsat least one molded coalescer module, wherein the plurality of coalescerelements are configured to operate simultaneously as a coalescer and aflow straightener, and wherein said at least one coalescer modulecomprises a complete transformer molded into the molded coalescermodule, and wherein said transformer comprises a primary low voltagewinding terminal accessible from outside of the coalescer module and ahigh voltage secondary winding insulated from the mixture of fluids. 2.The device according to claim 1, wherein the electrodes compriseelongated plates that are common for all coalescer elements in a row ofthe matrix, and shared between adjacent rows.
 3. The device according toclaim 1, wherein the device includes a central module being supplied byvoltage, wherein the matrix of the coalescer elements is arranged in aplurality of coalescer modules, mounted on opposite sides and adjacentto said central module, and wherein electric energy being transferred byan inductive coupling from the central module to the electrodes of thecoalescer modules.
 4. The device according to claim 3, wherein thecentral module includes a plurality of primary transformer windings andhalf cores, wherein each of said coalescer modules includes acorresponding secondary transformer winding and a half core, and whereinthe half cores in the coalescer modules and in the central moduleinductively complete a magnetic circuit allowing electrical energy to betransferred to the electrodes.
 5. The device according to claim 4,wherein the secondary winding and the coalescer elements for each moduleare molded in an insulating material, forming integrated modules.
 6. Thedevice according to claim 1, wherein the device includes a centralmodule being supplied by voltage, wherein the matrix of coalescerelements are arranged in a plurality of coalescer modules, mounted onopposite sides and adjacent to said central module, and wherein electricenergy is transferred by a capacitive coupling from the central moduleto the electrodes of the coalescer modules.
 7. The device according toclaim 6, wherein the central module includes a plurality of primarycapacitor plates, wherein each of said coalescer modules includescorresponding secondary capacitor plates, wherein the plates in thecoalescer modules and in the central module are mounted adjacent to eachother and separated by an insulating layer, and wherein the platescomplete a capacitive coupling allowing electrical energy to betransferred to the electrodes.
 8. The device according to claim 7,wherein the secondary capacitor plates and the coalescer elements foreach module are molded in an insulating material, forming integratedmodules.
 9. The device according to claim 1, wherein the tubularchannels are 5-30 mm in diameter and 100-500 mm long.
 10. The deviceaccording to claim 9, wherein channels are inclined downwards in theflow direction.
 11. The device according to claim 1, further comprisinga perforated metal plate mounted upstream of the matrix of coalescerelements and covering substantially the entire cross section of thevessel.
 12. The device according to claim 11, wherein said plate isgrounded to the separator vessel.
 13. An arrangement for promotingelectrostatic coalescence of a first conductive fluid emulsified in asecond fluid flowing through a vessel comprising an inlet and at leastone outlet, comprising: at least two devices arranged in series in theflow direction, each device comprising a plurality of parallel tubularelectrostatic coalescer elements extending in a flow direction, eachcoalescer element comprising an insulating tubular channel through whichthe fluids are to flow, and each coalescer element further comprising apair of external electrodes arranged outside of and adjacent to theinsulating tubular channel, the pair of electrodes having oppositepolarity and electrically insulated from fluids outside of the channels,wherein said interacting pair of electrodes have opposite polarity areseparated by a gap and extend in parallel in the length direction ofsaid channel, wherein the plurality of coalescer elements are arrangedin a matrix of coalescer elements over a cross sectional area of saidvessel such that the tubular channels are parallel to each other,wherein the matrix of coalescer elements forms at least one moldedcoalescer module, wherein the pluarality of coalescer elements areconfigured to operate simultaneously as a coalescer and a flowstraightener, wherein external electrodes at a same vertical position intwo adjacent arranged devices are of opposite polarity, and wherein saidat least one coalescer module comprises a complete transformer moldedinto the molded coalescer module, and wherein said transformer comprisesa primary low voltage winding terminal accessible from outside of thecoalescer module and a high voltage secondary winding insulated from themixture of fluids.
 14. An arrangement for promoting electrostaticcoalescence of a first conductive fluid emulsified in a second fluidflowing through a vessel comprising an inlet and at least one outlet,comprising: at least two devices arranged in series in the flowdirection, each device comprising a plurality of parallel tubularelectrostatic coalescer elements extending in a flow direction, eachcoalescer element comprising an insulating tubular channel through whichthe fluids are to flow, each coalescer element further comprising a pairof interacting pair of external electrodes arranged outside of andparallel to the insulating tubular channel, the interacting pair ofelectrodes having opposite polarity, are separated by a gap, extend inparallel in the length direction of said insulating tubular channel, andare electrically insulated from fluids outside of the channels, whereinthe coalescer elements are arranged in a matrix over a cross sectionalarea of said vessel such that the tubular channels are parallel to eachother, wherein the matrix forms at least one molded coalescer module,and wherein the coalescer elements are configured to operatesimultaneously as a coalescer and a flow straightener, wherein externalelectrodes at a same vertical position in two adjacent arranged devicesare of a same polarity, and wherein said at least one coalescer modulecomprises a complete transformer molded into the molded coalescermodule, and wherein said transformer comprises a primary low voltagewinding terminal accessible from outside of the coalescer module and ahigh voltage secondary winding insulated from the mixture of fluids.